A fluid dynamic bearing device relatively rotatably supports a shaft member by a dynamic pressure effect of a fluid caused in a bearing clearance. Recently, by taking advantage of its excellent rotational accuracy, high-speed rotation property, quietness, and the like, the fluid dynamic bearing device is suitably used as a bearing device for a motor to be mounted to various electrical apparatuses such as information equipment. Specifically, the fluid dynamic bearing device is used as a bearing device for a spindle motor mounted on information equipment including magnetic disk devices such as HDD, optical disk devices such as CD-ROM, CD-R/RW, and DVD-ROM/RAM, and magneto-optical disk devices such as MD and MO, and as a bearing device for motors such as a polygon scanner motor for a laser beam printer (LBP), a color wheel motor for a projector, or a fan motor.
For example, as a fluid dynamic bearing device incorporated into the spindle motor for an HDD, there is known one having a structure in which both a radial bearing portion for supporting the shaft member in a radial direction and a thrust bearing portion for supporting the shaft member in a thrust direction are structured by a dynamic pressure bearing. In this case, in one of an inner peripheral surface of a bearing sleeve and an outer peripheral surface of a shaft member opposed thereto, a dynamic pressure groove serving as a dynamic pressure generation portion is formed, and a radial bearing portion is often formed in a radial bearing clearance therebetween. Further, in one of a one end surface of a flange portion provided to the shaft member and an end surface of the bearing sleeve opposed thereto, the dynamic pressure groove is formed, and a thrust bearing portion is often formed in a thrust bearing clearance therebetween (for example, see Patent Document 1: JP 2003-239951 A).
The dynamic pressure groove is formed, for example, in a herringbone shape or a spiral shape, or a multi-arc or step-like arrangement in an outer peripheral surface of the shaft member. As methods of forming the dynamic pressure groove of this type, there are known, for example, cutting (see, for example, Patent Document 2: JP 08-196056 A) and etching (see, for example, Patent Document 3 JP 06-158357 A).
Further, as a method of forming the dynamic pressure groove, which enables reduction in material cost and machining cost compared to the cutting and etching, there is known rolling, for example. In this case, by applying predetermined heating after the rolling, a surface hardness of the material is often increased (see, for example, Patent Document 4 JP 07-114766 A).